At first glance the notion of wave-particle duality is strange indeed. You’ve likely learned about waves before and know them to be a disturbance in a medium, and you’ve likely learned about particles, which are discrete physical objects. So the idea that some things have properties of both might seem not only strange, but physically impossible.

This article will introduce you to the idea of wave-particle duality and give an overview of how the concept emerged and how it turns out to be an excellent description of reality in many cases, especially in the realm of quantum physics.

Let’s begin by reviewing what constitutes a wave. A wave is defined as a disturbance in a medium that propagates from one location to another, transferring energy in the process, but not transferring mass.

In the medium through which the wave moves, the individual molecules simply oscillate in place. A good example of this is a crowd in a stadium doing “the wave.” Each individual simply stands up and sits down, oscillating in place, while the wave itself travels around the entire stadium.

Wave properties include wavelength (the distance between wave peaks), frequency (the number of wave cycles per second), period (the time it takes for one complete wave cycle and velocity (how fast the disturbance travels).

Particles are distinct physical objects. They have a well-defined position in space, and when they move from one location to another, not only do they transfer energy, but their own mass as well.

Unlike waves, they do not need a medium through which to move. It also doesn’t make sense to describe them with a wavelength, frequency and period. Instead, they are usually described by their mass, position and velocity.

When the phenomenon of light was first being studied, scientists disagreed as to whether it was a wave or a particle. Isaac Newton's corpuscular description of light contended that it acted as a particle, and he developed ideas that explained reflection and refraction within this framework, although some of his methods didn’t quite seem to work.

Christiaan Huygens disagreed with Newton and used wave theory to describe light. He was able to explain reflection and refraction by treating light as a wave.

Thomas Young’s famous double-slit experiment, which demonstrated interference patterns in red light associated with wavelike behavior, also supported wave theory.

The debate as to whether light was a particle or a wave seemed to get resolved when James Clerk Maxwell came on the scene and described light as electromagnetic waves via his Maxwell’s equations.

But it soon became apparent that the wave nature of light did not account for all observed phenomena. The photoelectric effect, for example, could only be explained if light was treated as a particle – acting as single photons or light quanta. This idea was put forth by Albert Einstein, who won a Nobel Prize for it.

Thus was born the notion of wave-particle duality. Light could only be truly explained if it was treated as a wave in some situations and as a particle in others.

Here’s where things get even more strange. Not only does light display this duality, but it turns out matter does as well. This was discovered by Louis de Broglie.

This duality cannot be seen at all on a macroscopic scale, but when it comes to working with elementary particles, they sometimes seem to act as particles and other times as waves, with their wavelength equal to the associated de Broglie wavelength.

This notion led to the development of quantum mechanics, which describes particles with wave functions, which can then be understood in terms of the Schrodinger equation.